1. Field of the Invention
The present invention relates to a diaphragm control device for use in a camera which performs TTL (through-the-lens) light measurement while stopping down a diaphragm, and more particularly it pertains to a type of diaphragm control device which determines a diaphragm aperture size by suspending the diaphragm stopping-down operation when a light measuring value indicates its predetermined relationship with a preset value.
2. The Prior Art
Diaphragm control devices for cameras which perform TTL light measurement while stopping down a diaphragm are of course well known. The operation of such a known device is represented in FIG. 1 of the attached drawings. In FIG. 1 the dashed-dot line Vo shows the changes in the TTL light measuring value when the diaphragm is stopped down from the fully open aperture to the minimum aperture size, with the scene brightness remaining unvaried. In this case, the changes in Vo are, in other words, those in the aperture size, which can therefore be determined when the diaphragm stopping-down operation is suspended at point a, at which time Vo reaches predetermined value V.sub.t.
However, when a scene is placed under illumination, for example, using AC light, scene brightness varies timewise in accordance with the changes in the illuminous source light. In such situations, the changes in the TTL light measuring value caused by the diaphragm stopping-down operation are as shown by broken line V.sub.1 in FIG. 1 (wherein the means value of V.sub.1 is equal to Vo). It can be seen, for example, that the TTL light measuring value V.sub.1 may reach a level of reference signal V.sub.t at point b in FIG. 1, at which time the diaphragm stopping-down operation will be suspended. Accordingly, the diaphragm will unfortunately be stopped down only up to the aperture size corresponding to point c in FIG. 1, whereby the actually determined aperture size will be left larger than the aperture size which would be appropriate for the scene brightness.
In addition, it should be noted that cutting off the AC component of a light measuring signal by means of a low-pass filter or the like would result in the cut-off of even signal changes due to the diaphragm stopping-down operation, and thus the diaphragm control device will not operate properly at all.
It is, accordingly, an object of the present invention to provide a diaphragm control device of the above-described type which is improved insofar as it can automatically determine the diaphragm aperture with a minimum error even when alternative fluctuation components are involved in the scene brightness.